Isobutylene-isoprene polymers have been well known since the 1930s. They have good impermeability to air and a high level of damping when stretched or compressed. These polymers are used extensively throughout the tire and pharmaceutical industries. The copolymers are made by a cationic slurry polymerization process at approximately −95° C. using a catalyst comprising a Lewis Acid and an initiator. Lewis Acids such as the aluminum alkyls and aluminum chloride are used extensively in both laboratory experiments and commercial scale production. Initiators such as water and anhydrous HCl are used extensively.
Industry has generally accepted widespread use of a slurry polymerization process to produce butyl rubber, polyisobutylene, and other similar polymers in a diluent that dissolves the monomers and catalysts but not the polymer product. For manufacturing traditional butyl rubbers, i.e., isobutylene-isoprene copolymers, the slurry polymerization process offers a number of other advantages such as an achievable polymer concentration of approximately 26% to 37% by volume in the reaction mixture, as opposed to the concentration of only about 8% to 12% in solution polymerization. An acceptable relatively low viscosity of the polymerization mass is obtained enabling the heat of polymerization to be removed more effectively by surface heat exchange. Potential challenges associated with slurry polymerization are discussed in U.S. Pat. No. 6,939,933.
Isobutylene-para-methylstyrene (IPMS) polymers are also known. They are made in a similar process to isobutylene-isoprene polymers using similar initiation systems and are also used in the tire and pharmaceutical industries. However, there are a number of difficulties with IPMS polymerization, as compared to isobutylene-isoprene copolymerization, and these difficulties are exacerbated when using higher levels of para-methylstyrene (PMS) co-monomer. These difficulties include: instability of reaction temperatures and flash gas (reactor liquid composition); instability of Mooney viscosity control; lower than desirable conversion of monomer to product; higher than desirable warm-up rates due to rubber fouling, particularly around the reactor circulation pump; lower than desirable operability limitation on slurry concentrations; shorter reactor run lengths under comparable conditions; higher slurry viscosity under comparable conditions; and poorer and more erratic response of reactor to control parameters. Because of these difficulties it has historically been much more difficult and costly to produce IPMS copolymers than conventional isobutylene-isoprene copolymers. Currently, these undesirable process characteristics are managed by limiting throughput, PMS content, or a combination of the two.
Commercially, the production of IPMS copolymers is limited to slurry concentration of about 20 wt % polymer. This value is substantially lower than that observed for traditional isobutylene-isoprene copolymers, which can be operated at the above noted 26 to 37 wt %. The operating limits of IPMS polymerizations appear to stem from high slurry viscosity and the resulting poor heat transfer. Additionally, mass fouling is often experienced during initial polymerization. Methods to improve the concentration and stability of IPMS slurries could increase reactor productivity and decrease the cost of producing such polymers.